update elements and fix some bugs

lib/src/iron-flex-layout/iron-flex-layout.html

Index: lib/src/iron-flex-layout/iron-flex-layout.html
diff --git a/lib/src/iron-flex-layout/iron-flex-layout.html b/lib/src/iron-flex-layout/iron-flex-layout.html
index b7d1f087626047491d5c25d1a7e0b2c8dcc08742..c93795848f047d20b8fedddebb110f1ffc049e87 100644
--- a/lib/src/iron-flex-layout/iron-flex-layout.html
+++ b/lib/src/iron-flex-layout/iron-flex-layout.html
@@ -10,6 +10,20 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
 
 <link rel="import" href="../polymer/polymer.html">
 
+<!--
+The `<iron-flex-layout>` component provides simple ways to use [CSS flexible box layout](https://developer.mozilla.org/en-US/docs/Web/Guide/CSS/Flexible_boxes), also known as flexbox. This component provides two different ways to use flexbox:
+
+1. [Layout classes](https://github.com/PolymerElements/iron-flex-layout/tree/master/classes). The layout class stylesheet provides a simple set of class-based flexbox rules. Layout classes let you specify layout properties directly in markup.
+
+2. [Custom CSS mixins](https://github.com/PolymerElements/iron-flex-layout/blob/master/iron-flex-layout.html). The mixin stylesheet includes custom CSS mixins that can be applied inside a CSS rule using the `@apply` function.
+
+A complete [guide](https://elements.polymer-project.org/guides/flex-layout) to `<iron-flex-layout>` is available.
+
+@group Iron Elements
+@pseudoElement iron-flex-layout
+@demo demo/index.html
+-->
+
 <style>
   /* IE 10 support for HTML5 hidden attr */
   [hidden] {